During the 1980s Raman Scattering in fibers was demonstrated by Lin, Stolen, and other co-workers of AT&T Bell Laboratories in Holmdel, N.J. using Raman lasers operating between 0.3 to 2.0 μm. In the early years of the Raman fiber before extensive work had begun, no one perceived that a Raman fiber could be pumped by a practical semiconductor laser-based source or that an efficient CW-pumped Raman Fiber Laser was possible. However, with the development of Cladding-pumped Fiber Lasers and Fiber Bragg Gratings, diode-laser-based CW Raman Fiber Lasers have been made efficient, emitting at various wavelengths throughout the infrared spectrum a reality. (See van Gisbergen et al. (1996) Chem. Phys. Lett. 259: 599-604.)
Raman spectroscopy is a powerful optical technique for detecting and analyzing molecules. Its principle is based on detecting light scattered off a molecule that is shifted in energy with respect to the incident light. The shift, called Raman shift, is characteristic of individual molecules, reflecting their vibrational frequencies that are like fingerprints of molecules. As a result, the key advantage of Raman spectroscopy is its molecular specificity while its main limitation is the small signal due to low quantum yield of Raman scattering. One way to enhance the Raman signal is to tune the excitation wavelength to be on resonance with an electronic transition, so called resonance Raman scattering. This can usually produce an enhancement on the order of 102-103 fold.
Another technique to enhance Raman scattering is surface enhancement by roughened metal surfaces, notably silver and gold, that provides an enhancement factor on the order of 106-108. This is termed surface enhanced Raman scattering (SERS). Similar or somewhat larger enhancement factors (˜108-1010) have been observed for metal, mostly silver or gold, nanoparticles.
In the last few years, it has been shown that an even larger enhancement (˜1010-1015) is possible for aggregates of metal nanoparticles (MNPs), silver and gold. The largest enhancement factor of 1014-1015 has been reported for rhodamine 6G (R6G) on single silver nanoparticle aggregates. This huge enhancement is thought to be mainly due to significant enhancement of the local electromagnetic field of the nanoparticle aggregate that strongly absorbs the incident excitation light for the Raman scattering process. With such large enhancement, many important molecules that are difficult to detect with Raman normally can now be easily detected. This opens many interesting and new opportunities for detecting and analyzing molecules using SERS with extremely high sensitivity and molecular specificity.
SERS can also be developed into a molecular imaging technique for biomedical and other applications. Existing Raman imaging equipment should be usable for SERS imaging. SERS will provide a much-enhanced signal and thereby significantly shortened data acquisition time, making the technique practically useful for medical or other commercial and industrial applications including chip inspection or chemical monitoring. SERS is also useful for detecting other cancer biomarkers that can interact or bind to the MNP surface. For example, Sutphen et al. have recently shown that lysophospholipids (LPL) are potential biomarkers of ovarian cancer (Sutphen et al. (2004) Cancer Epidemiol. Biomarker Prev. 13: 1185-1191).
Photonic crystal fibers have been developed that can detect, identify, and quantify ultra small quantities of analytes in air and aqueous samples. In one example of the prior art, Du and Sukhishvili disclose a sensor comprising a photonic crystal fiber having an air hole cladding with functionalized air holes (Du and Sukhishvili, US Publication Number US 2007/0020144 A1, published 25 Jan. 2007). The photonic crystal fiber disclosed by Du and Sukhishvili comprises a solid core photonic crystal fiber; of note, Du and Sukhishvili described that “(c)omparison of FIG. 18 with FIG. 16 shows that the Ag nanoparticles 82 are present at a much lower density than the Ag nanoparticles 74 of the previous experiment. It is also apparent that the Ag nanoparticles 82 are much larger than the nanoparticles 74, and would, therefore, be less suitable for enhancement of SERS spectra” and that “(t)he moderate signals detected from adsorbed Rh6G (rhodamine 6G) in no salt aqueous solution were highly prone to fast photodegradation, and in a typical experiment, a SERS signal was not detectable after a 1 minute exposure of the substrate to 532 nm 10 mW laser radiation” (Du and Sukhishvili, paragraphs 59 and 52, respectively).
Others have also disclosed photonic crystal fibers, for example Konorov et al. (2005, Optics Express, 13: 3454-3459) and Konorov et al. (2006, Optics Lett., 31: 1911-1913). Konorov et al. (2005) disclose multicore hollow photonic crystal fibers of fused silica or soft glasses having inner diameters of the hollow core of about 2.5 μm and 3 μm or about 3 μm and 3.5 μm, respectively. Konorov et al. (2006) disclose hollow photonic crystal fibers with inner diameters of between about 8.6 μm and 9.5 μm and an outer diameter of 84 μm. Yan et al. also disclosed a novel hollow core photonic crystal fiber surface-enhanced Raman probe in Yan et al., (2006) (Yan et al. (2006) Appl. Phys. Lett. 89:204101).
The original single multimode SERS fiber probe was demonstrated in 1991 by Mullen et al. (Mullen and Carron (1991) Anal. Chem. 63: 2196). In the following years, studies involving different kinds of fiber tips were tested, such as flat, angled and tapered fibers (Viets and W. Hill (2000) J. Raman Spec. 31: 625; Viets and Hill (2001) J. Phys. Chem. B 105: 6330; and Viets and Hill (1998) Sens. Actuators B-Chem. 51: 92). Although they were easy to implement, the small number of SERS substrate particles in the active region limited the sensitivity of these sensors. In order to involve more particles in the SERS activity, hollow core photonic crystal fiber (HCPCF) and liquid core photonic crystal fiber (LCPCF) were tested recently (see Zhu et al. (2006) Opt. Exp. 14: 3541; Yan et al. (2006) Appl. Phys. Lett. 89: 204101; and Zhang et al. (2007) Appl. Phys. Lett. 90, 193504. High sensitivity, and low fiber SERS background show a promising future of PCF sensors. However, the wavelength sensitive nature of HCPCFs limits the application of a HCPCF to a single excitation wavelength and the cost of PCFs is still high. While normal fibers are lower in cost, their sensitivities are somewhat limited, often due to the background Raman scattering from the fiber itself. Therefore, it is highly desired to improve the detection sensitivity of SERS sensors based on conventional fibers. Fiber SERS sensors with high sensitivity, remote sensing capability, and low cost will find potential applications in medical, environmental, food detection, and toxin identification.
For many practical applications, for example SERS and optical fibers, it is highly desirable to narrow the distribution of size/shape of nanoparticle aggregates. For SERS in particular, the incident light has to be on resonance with the substrate absorption. Only those nanoparticle aggregates that have resonance absorption of the incident light are expected to be SERS active. It is thus extremely beneficial to have a narrow size/shape distribution and thereby narrow optical absorption.
Fluorescent nanoparticles (quantum dots (QDs) such as semiconductor quantum dots, SQDs) have been used recently as fluorescent biological markers and have been found to be extremely effective. They offer advantages including higher stability, stronger fluorescence, tunability of color, and possibility of optical encoding based on different sized or colored SQDs.
Metal nanoparticles have been recognized for their unique optical properties that could be exploited in optoelectronic devices. Nanoparticle systems composed of gold, for example, have distinct optical properties that make them amenable to study by Raman scattering. The Raman spectrum of the adsorbed species is significantly enhanced by 10 to 15 orders of magnitude when the metal nanoparticles have aggregated, leading to enhanced electromagnetic field effects near the surface that increases the Raman scattering intensity. The greater sensitivity found in the surface enhanced Raman scattering (SERS) of metal nanoparticle aggregates facilitates the detection and analysis of a whole host of molecules that were previously difficult to study.
Wang et al. disclose a method of using SQDs (dye-conjugated CdTe nanoparticles, CT-NPs) to detect interactive binding between Ag-CT-NPs and Ab-CT-NPs (Wang et al. (2002) NanoLett. 2: 817-822). The interactions were determined by differential quenching or enhancement fluorescence activity of two different sized SQDs (red or green) measured during the analysis.
The chemical methods used historically for the production of gold nanoparticle aggregates (GNAs) results in a wide distribution of aggregate size. This distribution leads to a broadened absorption spectrum. Accordingly, researchers have attempted to narrow the lineshape of the spectral peak due to the aggregates by homogenizing the size of the GNAs after they have been produced. By eliminating certain ranges of aggregate size, absorption spectrum peaks should narrow appreciably and concomitantly increase in intensity, resulting in more sensitive detection. Previous attempts to select for a narrow size range of aggregates have employed mechanical techniques such as passing a solution of aggregates through a filter. For example, Emory & Nie have employed size-selective fractionation using membrane filters to select for optically active silver nanoparticles (Emory and Nie, (1997) J. Phys. Chem. B, 102: 493-497).
The use of SERS for analyte detection of biomolecules has been previously studied. U.S. Pat. No. 6,699,724 to West et al. describes a chemical sensing device and method (nanoshell-modified ELISA technique) based on the enzyme-linked immunoadsorbant assay (ELISA). The chemical sensing device can comprise a core comprising gold sulfide and a surface capable of inducing surface enhanced Raman scattering (SERS). In much of the patent disclosure, the nanoparticle is disclosed as having a silica core and a gold shell. The patent discloses that an enhancement of 600,000-fold (6×105) in the Raman signal using conjugated mercaptoaniline was observed.
In the nanoshell-modified ELISA technique, antibodies are directly bound to the metal nanoshells. Raman spectra are taken of the antibody-nanoshell conjugates before and after the addition of a sample containing a possible antigen, and binding of antigen to antibody is expected to cause a detectable shift in the spectra.
The conjugation of quantum dots to antibodies used for ultrasensitive nonisotopic detection for use in biological assays has also been studied. U.S. Pat. No. 6,468,808 B1 to Nie et al. disclosed an antibody is conjugated to a water-soluble quantum dot. The binding of the quantum dot-antibody conjugate to a targeted protein will result in agglutination, which can be detected using an epi-fluorescence microscope. In addition, Nie et al. described a system in which a quantum dot is attached to one end of an oligonucleotide and a quenching moiety is attached to the other. The preferred quenching moiety in the Nie patent is a nonfluorescent organic chromophore such as 4-[4′-dimethylaminophenylazo]benzoic acid (DABCYL).
Raman amplifiers are also expected to be used globally as a key device in next-generation optical communications, for example, in wavelength-division-multiplexing (WDM) transmission systems. Raman scattering occurs when an atom absorbs a photon and another photon of a different energy is released. The energy difference excites the atom and causes it to release a photon with low energy; therefore, more light energy is transferred to the photons in the light path.
There is therefore a need in the art for use in the biomedical analytical industries and the optical communications industries to provide more sensitive compositions and devices that are inexpensive to manufacture and easy to use.